Flocculation processes

ABSTRACT

When flocculating an aqueous suspension of suspended solids using a high molecular weight synthetic polymeric flocculant, the shear stability of the flocs is increased if the polymeric material includes polymeric particles of below 10 μm dry size. The flocculated solids can therefore be subjected to shear without increasing the amount of discrete suspended solids in the aqueous medium and generally they are subjected to shear by shearing the aqueous medium containing them, either before dewatering, generally on a centrifuge, piston press or belt press, or by continuously agitating them, for instance in a chemical reaction medium. The polymeric material is generally formed by mixing into water polymeric particles made by reverse phase or emulsion polymerization in the presence of added cross linking agent. Alternatively, particles insolubilized by insoluble monomer may be used. A reverse phase dispersion of water soluble polymer may be used if the particles remain undissolved, e.g. if they are added in the absence of an oil in water emulsifying agent.

When synthetic polymers of water soluble monomers or monomer blends werefirst introduced as flocculants, in the early to mid-1950's, maximummolecular weights were relatively low compared to the present day. Theinitial polymers typically had molecular weights well below 500,000 andthus were of a value comparable to the molecular weight now associatedwith coagulants, rather than high molecular weight flocculants. Theselow molecular weights were probably caused by the presence of chaintransfer agents and other impurities in the monomer or polymerisationmixture.

It was recognised that the polymers had to be in solution and if thepolymers were not, despite their low molecular weight, spontaneouslysoluble in water (for instance due to excessive cross linking) it wasappreciated to be necessary to homogenise them so as to put them intosolution. For instance Miller described in U.S. Pat. No. 3,021,269ultrasonic degradation of a highly cross linked insoluble polymer gel"having almost infinite molecular weight" to render it water soluble asa result of severing the polymeric structure. The end products werealways of relatively low molecular weight and the highest quotedmolecular weight for the end product is 630,000 and the highestintrinsic viscosity (IV) 2.54 dl/g.

Similarly, Goren described in a 1954 patent application (published asU.S. Pat. No. 3,235,490) dispersing various polymer gels into waterusing a Waring Blendor. Many of the gels were cross linked spontaneouslyor by the addition of cross linking agent and the cross linking appearsto have caused the formation of some wholly insoluble, non-swellable,polymer that settled out of solution. Solutions of the polymers werealso subjected to homogenisation in a hand homogeniser and it wasobserved that the effect on agglomeration performance of thishomogenisation is drastic, with most of the products being useless afterhomogenisation. Again, all the polymers were of very low molecularweight as is indicated by the fact that the highest specific viscosity(measured by a capilliary flow viscometer at 34° C. on a 0.5% solutionin deionised water) is quoted as 0.77. This compares to values of wellover 100, and usually over 1000, for modern high molecular weightflocculants.

Some polymers having molecular weights typical of those described byMiller and Goren can be used as coagulants, for instance for coagulatingvery fine suspended solids, e.g., for clearing turbidity or removingcolour from aqueous solutions. For instance, typical modern polymercoagulants have a molecular weight of up to about 500,000. Typicalpolymer coagulants may be formed by reaction of epichlorhydrin withdimethylamine. Since the resultant linear product may have extremely lowmolecular weight, it is known to include ethylene diamine in order toincrease molecular weight by cross linking without rendering the polymerinsoluble.

Goren postulated that agglomeration involved electrostatic attractionfollowed by a sweeping action of a filamentary network of the crosslinked polymer. This mechanism has come to be recognised as theclassical mechanism of coagulating turbidity and colour, namely veryfine suspended solids. Goren made his polymer by bulk polymerisationfollowed by comminution and showed that his aqueous compositions tendedto be non-homogeneous, in that there was a tendency for insolublepolymer to precipitate from the solution. Goren warned against crosslinking too much and indicated that the optimum was the level at whichthe polymer is still readily dispersible in water. Since Goren waspostulating a sweeping action by filamentary molecular networks thisindicates that his dispersibility had to be on a molecular scale, i.e.,true solution. Goren warned that the agglomerating effect of the polymercan be destroyed by homogenising it (column 13 line 74).

In contrast to these low molecular weight polymer coagulants, modernflocculants (for flocculating suspended solids such as sewage) arelinear polymers of very high molecular weight. Most have an intrinsicviscosity above 4 and often above 10. The polymers have to be linearsince cross linking renders them ineffective and often insoluble,although trivial amounts of cross linking may exist without detractingfrom the polymer properties (see for instance U.S. Pat. No. 3,557,061column 3 line 35).

Whether or not a high molecular weight polymer is suitable for use as aflocculant is determined in part by observing the rheology of aqueouscompositions of the polymer. Satisfactory polymers give a "long" or"stringy" rheology. This is recognised in the art and can bedemonstrated in that when a 1% stable homogeneous composition of thepolymer in deionised water is formed by conventional techniques, such asby stirring using a slowly rotating stirrer followed by ageing, and aglass rod is manually pulled endwise out of the solution the rod draws along thread of composition with it. The thread is generally at least 5cm long and the polymer can then be described as having a rheology of 5cm. Often the rheology is above 10 cm. If, in contrast to this, thepolymer gives a "short" or "granular" rheology (i.e., in the above testthe rod pulls substantially no thread, for instance below 5 cm and oftenbelow 2 cm, of composition), the polymer will be rejected and will notbe used as a flocculant. Experience has shown that polymers giving thisshort rheology are unsatisfactory in conventional flocculation processessince it indicates a high degree of cross-linking and/or a low molecularweight. The short polymers can also be characterised as non-filmforming, in that when an aqueous composition is dried it does not form afilm.

Similarly the polymer is rejected if it has a large particle size and iscross linked sufficient to ensure that insoluble solid polymer does notgo into stable suspension in the aqueous composition.

The stable homogeneous composition is stable in the sense that thepolymer is in full equilibrium with the water, i.e., it has reached itsultimate degree of solution or swelling, for instance as a result ofageing for two hours or more. It is homogeneous in the sense that thepolymer remains uniformly dispersed throughout the composition (usuallyin the total absence of dispersing agent although minor amounts may bepresent as a result of the manufacture of the polymer) with no tendencyfor material to precipitate from the composition on standing for a fewdays.

The unsuitability as flocculants of short rheology polymers (as definedabove), and of polymers that do not go into stable suspension orsolution are well understood in the art. Very high molecular weight,linear, truly dissolved polymers are preferred.

Certain high molecular weight polymers, for instance polymers of Mannichbases, have a tendency to cross link spontaneously and acquire a rathershort or very short rheology or become totally insoluble. It frequentlyhappens that high molecular weight polymers are produced which haverheology that is shorter than is desirable. Polymers of very shortrheology (below 2 cm), or that are insoluble, are rejected. Polymerswith longer, but still rather poor, rheology may be used under the sameconditions as if they had the desired long rheology but this leads topoor performance properties.

In GB Pat. No. 1,579,007 it is alleged that high molecular weightcationic flocculants give optimum performance when the polymers have acationicity value of at least 90% of the theoretical cationicity value.

Flocculant polymers may be made by reverse phase suspension or emulsionpolymerisation to a very small particle size. Before use, the resultantemulsion is added to water, generally in the presence of oil-in-wateremulsifying agent and usually with stirring, and allowed to form a truesolution before use. Thus the system is always allowed to go toequilibrium (i.e., a stable homogeneous composition), often shown byattainment of maximum viscosity, before it is added to the suspension.

The linear, high molecular weight flocculant polymers are used byforming, with ageing, a true aqueous solution of the polymer and dosingthis with minimum agitation into the suspension, followed by dewateringof the suspension. Optimum results require accurate dosing and theminimum of agitation during flocculation. If the dose is too low or toohigh flocculation is inferior. The optimum dose depends upon the contentof the suspension and so variations in it, for instance variations inthe metal content of industrial sewage effluent, can greatly affectperformance. The flocs are very sensitive to shear and agitation,especially if the dosage is not at an optimum, is likely to redispersethe solids as discrete solids. This is a particular problem when theflocculated solids are to be dewatered under shear, for instance on acentrifuge, because if dosage and other conditions are not optimum thecentrate is likely to have a high discrete solids content.

It would be desirable to provide a flocculation process in which thedosage of flocculant is less dose sensitive and the flocs are morestable to shear than with conventional dissolved high molecular weightflocculant polymers.

In the invention, an aqueous suspension of suspended solids isflocculated by adding a synthetic polymeric material to form an aqueousmedium containing flocculated solids and the process is characterised inthat at the time of addition to the suspension, the polymeric materialhas a specific viscosity above 10 (generally above 100), and comprisespolymeric particles having a dry size of below 10 μm, the polymericmaterial is added in a floc stabilising amount, and the flocculatedsolids are subjected to shear in the presence of the aqueous mediumsubstantially without increasing the amount of discrete suspended solidsin the aqueous medium.

Thus the invention is based on the discovery that the shear stability ofthe flocs can be increased by initiating, and usually completing,flocculation while some or all of the polymeric material is in the formof small particles rather than a true solution. For optimumflocculation, the dosage of the polymer should usually be greater thanthe optimum amount used with fully dissolved polymers but the process isnot so dose sensitive as with fully dissolved polymers.

The particulate material can be soluble. Thus a conventional reversephase emulsion of soluble polymer can be mixed direct into thesuspension or, usually, is diluted in the absence of oil-in-wateremulsifier and/or with insufficient stirring or ageing to form a truesolution, and is added to the suspension. This is in contrast toprevious experience where the aqueous composition was always allowed toage to maximum viscosity (true solution) before use since conventionallow shear flocculation (e.g., sedimentation) had shown this to beessential for satisfactory results.

It is often preferred that the polymeric material should compriseparticulate insoluble polymer. This may be insoluble due to theinclusion of insolubilising monomers or due to the provision of acontrolled degree of non-linearity in an otherwise soluble polymer.Commercially, the invention is best performed by making reproducibly apolymeric material having a controlled degree of non-linearity, which isused to flocculate an aqueous suspension and this is then subjected toshear without substantial redispersion of the solids to become discretesuspended solids. This procedure is in contrast to previous experiencewhere non-linearity may have occurred by accident and the polymer wasthen either rejected as being useless or was used in either aconventional low shear process or was used in an inadequate amount in ahigh shear process, with consequential redispersion of solids.

The shear to which the flocculated solids are subjected may be appliedonly during dewatering of the solids but preferably the flocculatedsolids are subjected to shear by shearing the aqueous medium containingthem. For instance instead of mixing the aqueous flocculant into thesuspension in conventional gentle manner, with little or no agitation ofthe flocs, in the invention the aqueous medium is preferably sheared bystirring sufficient to reduce floc size. This is particularly desirablewhen the aqueous suspension is viscous, e.g., it has a solids contentabove 3% by weight for primary or digested sludges or about 1.5% foractivated sludges. At these high solids contents, the flocs are likelyto be very large, for instance above 5 cm (and often the solids may gointo a substantially continuous floc) and the shear is preferably suchas to break this large floc structure down into flocs typically having asize in the range 2 to 20 mm. Because of the particulate nature of thepolymer, and appropriate choice of the amount of particulate polymer,this floc breakdown occurs without the amount of discrete suspendedsolids in the aqueous medium increasing substantially, or preferably atall, compared to the amount that is present if the suspension is notsheared.

After shearing, the medium may be dewatered. Dewatering may be bysedimentation or by drainage or vacuum filtration but a particularadvantage of the invention is that the floc structure can be veryeffectively dewatered under shear, and in particular on a centrifuge,piston press or belt press, to give very high recovery of solids, andvery low suspended solids in the filtrate.

Dewatering of flocculated solids can be conducted, especially atrelatively low solids concentrations, under shear, e.g., on acentrifuge, even if the flocculated aqueous medium is not firstsubjected to shear, but generally less effective dewatering is obtained.

The improved floc structure obtainable in the invention, compared to theuse of conventional dissolved flocculants, permits dewatering to anincreased solids content, thereby for instance reducing the amount ofenergy required for incinerating a sewage filter cake. The increasedfloc strength however is valuable in various other processes.

The flocs obtained in the process (preferably using insoluble polymer)can be continuously kept in suspension by agitation of the aqueousmedium without any substantial increase in the discrete suspended solidsin the aqueous medium. For instance, the polymeric material (usually asa stable homogeneous aqueous composition) may be added (optionally withapplied shear) to an aqueous suspension whilst it is being agitated andthis agitation may provide shear and may keep the resultant shearedflocculated solids in suspension. The continuous agitation may becontinued for several hours and usually for at least a day or severaldays, without substantial floc breakdown. This is of value, especiallywhen using anionic flocculants, for transporting inorganic or othersolids in a fluid medium, for instance by pipeline or in any process inwhich agitation is applied for prolonged periods, e.g., in chemical orbiochemical reactors.

The process is of particular value when the aqueous medium is a chemicalreaction medium and the solids are a catalyst for the reaction, since wehave surprisingly found that a stirred or otherwise agitated reactor canbe operated for prolonged periods with the catalyst in the form offlocs. This facilitates the separation of the aqueous medium from thecatalyst, for instance as the medium is withdrawn continuously orbatchwise from the reactor. This process is of particular value in thecatalytic hydrolysis of a nitrile to form an amide, for instance whenthe aqueous medium is an acrylonitrile hydrolysis reaction medium toform acrylamide. The catalyst is preferably a copper catalyst, forinstance of reduced copper oxide or, preferably, Raney copper.

Another process where the shear resistance of the flocs is desirable isin the formation of paper and paper products such as board, since theprocesses of the invention permit improved dewatering of cellulosic andother suspensions. In conventional paper production, it is generallynecessary to minimise the amount of shear to which the flocs aresubjected and so in practice the flocculant is added at the end of thepulp flow line, as late as possible before the drainage or otherdewatering stage. In the invention, however, it is possible, andfrequently desirable, to add the flocculant (preferably an insolublepolymer) at an early stage in the pulp flow line so that the act ofpumping the flocculated dispersion along the flow line towards thedrainage or other dewatering stage involves the application of shear tothe flocculated pulp, and this shear converts the flocs to medium orsmall size flocs substantially free of undesirable fines. A preferredprocess of the invention therefore comprises flocculating a cellulosicsuspension by addition of the polymeric material, usually as a stablehomogeneous aqueous composition and pumping the flocculated suspensionalong a flow line with sufficient shear to break down the flocs tosmaller, shear stable, flocs and then dewatering the suspension bydrainage or other suitable means. This process is of particular valuewhen cationic starch is also added to the dispersion since the overallprocess then gives an exceedingly good combination of paper strength andretention and dewatering properties. For this process, the flocculantpolymer is preferably an anionic polyacrylamide. Synergism appears toexist.

Another advantage of the invention is that the process is much less dosesensitive than when using truly dissolved flocculants and so there ismuch less risk of obtaining inferior performance due to under-dosing orover-dosing. Even after shearing the suspension, it is usually possiblyto obtain floc size that is much greater than is obtainable usingconventional dissolved flocculants. Because, at the optimum dose, thefloc size is much greater than is available conventionally this meansthat the dose can be varied above or below the optimum whilst stillobtaining improved results compared to those obtainable conventionally.

Although the invention can be used for flocculating a wide variety ofaqueous inorganic suspensions and aqueous organic suspension, especiallysewage, it is of particular value in the centrifugal dewatering ofmunicipal sewage that includes a significant industrial component,especially that includes variable amounts of metal. Best results aregenerally obtained when the flocculated aqueous medium is vigorouslystirred, so as to apply shear to the flocculated solids, before enteringthe bowl of the centrifuge.

The polymeric material can be dosed into the suspension in anyconvenient manner (e.g., a dispersion in oil could be metered carefullyinto the suspension) but is preferably added in the form of a diluteaqueous composition, typically having a concentration of 0.01 to 3%,generally 0.05 to 1%. It should preferably be a microdispersion. By thiswe mean that if a layer of this composition is allowed to dry,microscopic examination readily identifies discrete polymeric particles,optionally interconnected by a film of water soluble polymer. Often thecomposition does not form a film. The polymer particles must be below 10μm dry size, preferably below 2 μm, but preferably swell, e.g., to atleast twice their dry diameter, and often at least 5 times their drydiameter, in water.

When the polymer particles are insoluble, it is preferred for theaqueous composition to be a homogeneous stable composition as definedabove although the polymer particles can go to equilibrium with thewater in the flocculated suspension to some extent at least. If thepolymer particles are soluble, then they must be added to the suspensionbefore they dissolve and preferably initial flocculation is completebefore they can dissolved.

In order that the particles have the desired small size, they are bestprepared by emulsion or reverse phase polymerisation.

Although we believe it to be essential to include non-dissolvedparticles, it appears that dissolved polymeric material may alsocontribute to the invention and so the polymeric material that is addedto the suspension may include also dissolved linear polymer. When thepolymer particles are cross linked, it is generally preferred to providethis as a soluble component of the cross linked particles, so that upondispersing the particles in the aqueous composition the particles swelland the soluble component dissolves into the composition. However it isalso possible either to blend a dissolved polymer with a particulate(generally insoluble) polymer in the aqueous composition or to add thesepolymers sequentially to the aqueous suspension, the dissolved polymergenerally being added first. When the mixture of dissolved andparticulate polymers is made by blending polymers, the chosen polymersare usually co-ionic or one or both may be non-ionic, or they may becounter ionic. Usually the polymers are made from the same monomers, andoften differ only in the degree of cross linking.

The amount of dissolved polymer is usually from 0 to 50%, preferably upto 20%, preferably at least 10%, by weight total polymer, the balancebeing particulate.

The particles may be wholly insoluble, non-swellable, polymer particles.For instance, they may be formed of wholly water insoluble monomers or,more preferably, a blend of water soluble and water insoluble monomerssuch that the polymer is insoluble in water (generally at both high andlow pH values). Insolubility is often further increased by crosslinking.

Preferably, however, the particles are formed of a monomer or monomerblend that is soluble in the aqueous composition and the particles areeither soluble in the aqueous composition and are used before theydissolve, or, preferably, are cross linked sufficient that they areinsoluble in but swollen by the aqueous composition. This cross linking(which may be chain branching) may be brought about by controlledspontaneous conditions such as heating or irradiation, provided thedegree of chain branching or other cross linking is reproducible andcontrollable, but preferably is brought about by reaction of the monomeror monomer blend, or the final polymer, with a covalent or ionic crosslinking agent.

Cross linked polymer can be made by cross linking a preformed linearwater soluble polymer having a specific viscosity above 10 with a crosslinking agent, e.g., by mixing an aqueous solution of the polymer withcross linking agent whilst stirring with sufficient force to form ahomogeneous stable aqueous composition. If the stirring is inadequate,cross linked polymer will precipitate from the composition. If it isadequate, then the polymer will be broken up into sufficiently smallparticles, below 10 μm and preferably below 2 μm dry size, that theparticles will remain in stable homogeneous dispersion. The crosslinking agent for this purpose can be, for instance, formaldehyde orglyoxal or metal salts but preferably is a counterionic linear watersoluble polymer having specific viscosity above 10. Preferably bothpolymers have specific viscosity above 100. By selecting appropriateamounts of polymers having anionic and cationic groups, it is possibleto obtain a stable homogeneous aqueous composition of coprecipitated, orcross linked, polymer and, if desired, to leave an excess of a watersoluble polymer of one ionic type.

The preferred way of making the aqueous composition is by mixing intowater particles of polymeric material having dry size below 10, and mostpreferably below 2, μm and which have been made emulsion polymerisationor by reverse phase emulsion or suspension polymerisation of one or moremonoethylenically unsaturated monomers. The polymer may be soluble butis preferably insoluble as a result of a controlled addition ofcross-linking agent to the monomer or monomer blend, which is preferabywater soluble.

The monoethylenically unsaturated material may be contaminated with asmall amount of cross-linking agent and the amount of additionalcross-linking agent that is added will therefore be selected havingregard to this fact. Preferably the monoethylenically unsaturatedmaterial is as free of cross-linking agent as is commercially possible,for instance containing cross-linking agent in an amount that givescross linking or chain branching less than is given by ppm MBA (1 partmethylene bis acrylamide per million parts monomer). The amount of MBAthat is added is generally at least 0.1 or 0.2 ppm and below 100 ppm(based on monomer), generally 1 to 50 ppm. The precise amount willdepend upon the polymerisation and other processing conditions. Insteadof using MBA, cross-linking my be by equally effective amounts of otherdiethylenically unsaturated compounds such as ethylene glycold-acrylate, diacrylamide, cyanomethylacrlate, vinyloxyethylacrylate ormethacrylate and other means of cross linking, e.g., formaldehyde orglyoxal or metal salt addition. Preferably a water-soluble cross-linkingagent is used.

The degree of non-linearity can additionally be controlled by theinclusion of chain transfer agents in the polymerisation mixture. Theiruse, in combination with cross-linking agent, will tend to promote chainbranching rather than cross linking. Amounts may vary widely. Forinstance 1,000 to 5,000 ppm (based on monomer) of a moderate chaintransfer agent such as isopropyl alcohol may be suitable whilst muchlower amounts, typically 100 to 500 ppm, of more effective chainbranching agents such as mercaptoethanol are useful. Often, however,adequate results are obtained by conducting polymerisation underconventional conditons, without deliberate addition of chain transferagent, using commercially pure monoethylenically unsaturated monomertogether with the specified amount of MBA or other cross-linking agent.

Instead of insolubilising the polymer by cross linking, it may be formedfrom an insoluble monomer, or a monomer blend containing sufficientinsoluble monomer to insolubilise the polymer.

The monoethylenically unsaturated monomers may consist of one or moreionic monomers or a blend of ionic and non-ionic monomers. The monomerscan be allyl monomers but are generally vinyl, preferably acrylic.

Suitable non-ionic monomers are acrylamide, methacrylamide,N-vinylmethylacetamide or formamide, vinyl acetate, vinyl pyrrolidone,methyl methacrylate or other acrylic (or other ethylenicallyunsaturated) ester or other water insoluble vinyl monomers such asstyrene or acrylonitrile.

Suitable anionic monomers are sodium acrylate, methacrylate, itaconate,2-acrylamido 2-methyl propane sulphonate, sulphopropyl acrylate ormethacrylate or other water soluble forms of these or otherpolymerisable carboxylic or sulphonic acids. Sulphomethylatedacrylamide, allyl sulphonate or sodium vinyl sulphonate, may be used.

Suitable cationic monomers are dialkylaminoalkyl acrylates andmethacrylates, especially dialkylaminoethyl acrylate, and theirquaternary or acid salts, and dialkylaminoalkylacrylamides ormethacrylamides and their quaternary or acid salts for instancemethacrylamidopropyl trimethyl ammonium chloride and Mannich products,such as quaternised dialkylaminomethylacrylamides. Alkyl groups aregenerally C₁₋₄ alkyl.

The monomers can contain hydrophobic groups, e.g., as described in EPNo. 0172723A2, for instance on page 10 thereof. If the monomer is toimpart insolubility to the polymer, the ethoxy chain should be short orabsent, i.e., n=0. The allyl ether monomers are especially preferred.

The polymerisation conditions are preferably such that the polymer has,if uncross linked, a conventional flocculant high molecular weight of 5million to 30 million and an intrinsic viscosity of above 4, preferablyabove 6, e.g., up to 15 or 20 dl/g. If the polymer is cross linked, itis preferably polymerised such that it would have such molecular weightif it had been made in the absence of cross linking agent. However crosslinking will reduce the IV but the shearing may then cause the IV toincrease, as explained below. The specific viscosity of the polymer,measured as defined above, is generally above 100, preferably above 500and frequently above 1000.

The particle size in the emulsion or reverse phase polymerisationmixture may be controlled by the degree of shear applied to the monomersand by the possible presence of emulsifying agent. Emulsionpolymerisation may be utilised when polymerising, for instance, waterinsoluble monomers such as acrylic esters or water insoluble but acidsoluble monomers such as amines (the resultant polymer being distributedinto acidic aqueous composition) but generally reverse phase emulsion orsuspension polymerisation is utilised when the monomer or monomer blendis soluble in water. The aqueous monomer is emulsified into a suitablenon-aqueous liquid, generally in the presence of a water in oilemulsifier, generally in an amount below the critical micellconcentration. Emulsifiers, stabilisers, non-aqueous liquids and otherreverse phase polymerisation materials and process details are describedin, for instance, EP No. 0126528. The polymer particles may bedehydrated, for instance by subjecting the dispersion to azeotropicdistillation.

The liquid product resulting from the reverse phase polymerisation oremulsion polymerisation is generally used as such, without separation ofthe polymer particles, but if desired dried polymer particles may beseparated from the dispersion in known manner. Because these dryparticles will be very dusty, they should preferably be formed intopellets that will disintegrate upon addition to water.

The polymer-in-oil emulsion that results from reverse phasepolymerisation may be added to water to form the aqueous composition (orto the suspension) in the presence of oil-in-water emulsifier inconventional manner. However when the polymer is water-soluble, it ispreferred to make the addition in the absence of the emulsifier so thatthe rate of solution is slower. The reverse phase emulsion is preferablydehydrated.

The polymerisation conditions are preferably such that the polymerparticles resulting from the polymerisation have the desired controlleddegree of solubility but it is possible to produce polymer particlesthat are too highly cross linked and then to subject this polymer tosufficient shear to restore it to a desired, controlled, degree of crosslinking. This shear may be applied to the dispersion in which thepolymer particles are formed or, preferably, to the aqueous homogeneouscomposition. For instance when such a solution has short rheology, themixing may convert it to long rheology. These processes are described inour application Ser. No. 855,519 filed even date herewith.

When the polymeric material is cross linked and cationic, and inparticular when it is a copolymer of acrylamide with at least 5%, andpreferably at least 10%, by weight dialkylamino alkyl acrylate(generally as acid addition or quaternary ammonium salt), the degree ofnon-linearity is preferably such that the polymer has an ionic regain(IR) of at least 15%. IR is calculated as (x-y)/x×100 where x is theionicity measured after applying standard shear and y is the ionicity ofthe polymer before applying standard shear.

These values are best determined by forming a 1% composition of thepolymer is deionised water, allowing this to age for 2 hours and thenfurther diluting it o 0.1% active polymer. The ionicity of the polymer yis measured by Colloid Titration as described by Kock-Light LaboratoriesLimited in their publication 4/77 KLCD-1. (Alternatively the methoddescribed in BP No. 1,579,007 could possibly be used to determine y.)The ionicity after shear, x is determined by measuring by the sametechnique the ionicity of the solution after subjecting it to standardshear.

The shear is best applied to 200 ml of the solution in a substantiallycylindrical pot having a diameter of about 8 cm and provided in its basewith a rotatable blade about 6 cm in diameter, one arm of the bladepointing upwards by about 45 degrees and the other downwards by about 45degrees. The blade is about 1 mm thick and is rotated at 16,500 rpm inthe base of the pot for 10 minutes. These conditions are best providedby the use of a Moulinex homogeniser but other satisfactory conditionscan be provided using kitchen blenders such as Kenwood, Hamilton Beach,Iona or Osterizer blenders or a Waring Blendor.

In practice the precise conditions of shear are relatively unimportantsince, provided the degree of shear is of the same order of magnitude asspecified, it will be found that IR is not greatly affected by quitelarge changes in the amount, for instance the duration, of shear,whereas at lower amounts of shear (for instance 1 minute at 16,500 rpm)IR is greatly affected by small changes in shear. Conveniently,therefore, the value of x is determined at the time when, with a highspeed blade, further shear provides little or no further change inionicity. This generally requires shearing for 10 minutes, but sometimeslonger periods, e.g., up to 30 minutes with cooling, may be desired.

It should be understood that the defined shear is not shear that isapplied to the polymer solution or to the flocculated suspension duringthe flocculation process of the invention but is instead shear that isapplied as an analytical technique to permit definition of theproperties of the polymers that may be used in the invention.

When using cross-linked polymeric material, polymers having IR of 15%have a relatively low degree of non-linearity whilst those having IR 90%have a high degree of non-linearity. It is generally preferred for IR tobe below 80%; preferably below 70%, and usually below 60%. If IR is toolow, the invention may give inadequate benefit compared to conventionalpolymers and preferably IR is above 20%. Best results are generallyobtained at above 25%, preferably 30 to 60%.

It is desirable for the intrinsic viscosity to be as high as possiblebut satisfactory values of IV reduce as the value of IR increases.Generally IV=(100-IR)/a where a is below 20 and is generally below 15but is usually above 4. Generally a is in the range 6 to 14. Throughoutthis specification IV is measured at 25° C. in 3M NaCl according to themethod described in Encyclopedia of Polymer Science & Technology,Editors Mark and Gaylord, published John Wiley & Sons, 1971, Volume 14pages 717-740.

If the polymer is cross-linked, IV can be increased by the applicationof shear (as is also described in application Ser. No. 855,519 and thepolymeric material is preferably one whose IV can be above 4, andpreferably above 6, after the application of shear such as the standardshear described above.

The aqueous composition of the polymeric material may be combined withthe suspension that is to be flocculated by conventional methods ofblending but, as described above, shear is generally applied sufficientto reduce floc size.

The amount of polymer that has to be added for optimum floc stability isoften greater than the amounts conventionally used with highly solublepolymeric flocculants, usually at least 10% and often at least 20%greater than the amount that would be required when using aconventional, highly water soluble, substantially linear polymer.Suitable doses are in the range 0.01 to 3%, often 0.5 to 3%, by weightpolymer based on dry solids.

The amount that is required for adequate floc stability can be found byroutine experimentation and, for any particular flocculation process,polymer type and degree of shear, the amount of polymer will depend uponthe degree of swelling or solution of the polymeric material, e.g., thedegree of cross-linking. Generally the optimum amount increases withincreasing amounts of cross linking.

A convenient way for determining the optimum floc stabilising amount isto determine the dose that gives maximum floc size when the polymericmaterial is sheared into the suspension and the suspension is left tosettle. The optimum is the dosage that gives maximum floc size aftershear and in the invention, the dose that is applied is generally from50 to 150, preferably 70 to 110% of this optimum dose.

A particularly preferred process according to the invention comprisesproviding a homogeneous dilute aqueous composition of a reverse phasepolymerised, non-film-forming, cross linked, acrylamide copolymer withdialkylaminoalkyl(meth)acrylate (as acid salt or quaternary salt) havinga dry particle size below 10 μm (preferably below 2 μm), IR above 15 andIV=(100-IR)/a) where a is from 6 to 14, adding this composition tosewage sludge in an amount of 50 to 150% of the amount required formaximum floc size after shearing, subjecting the blended mixture to theshearing to reduce floc size substantially without increasing the amountof suspended solids, and dewatering the resultant aqueous medium on acentrifuge, piston press or belt press.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing is a graph of the results in Example 2.

The following are some examples.

In every example, the polymeric material had a specific viscosity wellabove 100 and a dry particle size below 2 μm and an aqueous compositionof the polymeric material gave a discontinuous, particulate film.

EXAMPLE 1

A copolymer of 58% acrylamide and 42% dimethylaminoethylacrylatequaternised with methyl chloride (DMAEA.MeCl) and having intrinsicviscosity 10 dl/g was prepared by reverse phase polymerisation, to givea particle size below 2 μm, followed by azeotropic distillation. It waslabelled Polymer AC and was provided as a 50% dispersion of polyme inoil. The monomers used were commercially pure monomers. Polymer BC wasformed by the same method but in the presence of 10 ppm MBA, and hadintrinsic viscosity 6.6 dl/g.

Each dispersion was mixed with water and allowed to age. A chosen amountof the resultant aqueous composition was stirred with an activatedsludge for 25 seconds using Triton WRC Standard Shear Test Stirrer andTimer Type 133/131 fitted with a marine blade to give extra shear. Thisresulted in flocculaton and in reduction of the floc size.

The flocculated suspension was dewatered on a laboratory centrifugeconsisting of a cylindrical solid bowl closed at its base and open atits top but with an inwardly extending lip around its periphery. Thebowl ran at 2,000 rpm and was, at this speed, filled with water (400ml). 400 ml of the flocculated sewage sludge was fed gradually into thebowl while spinning. Some of the solid was trapped in the bowl whilstthe remainder passed out in the overflow, as the centrate. Since theflocculate suspension is accelerated, in a very short period of time, to2,000 rpm, this centrifugal system of dewatering applies very high shearto the flocculated suspension. Best results are those wherein there ismaximum retention of solids in the bowl, with least solids content inthe centrifugate.

The dose of g/m³ and the suspended solids in the centrate (mg/l) whentreated with each of the polymers AC and BC are shown in Table 1a.

                  TABLE 1a                                                        ______________________________________                                        Dose            AC     BC                                                     ______________________________________                                        20              1148   1400                                                   30              1088   660                                                    50               667   368                                                    60              1863   244                                                    70              2227   342                                                    80              2670   402                                                    100             4627   626                                                    120             5372   726                                                    ______________________________________                                    

The cationicity regain of the polymers was recorded for 10 minutesshearing, as in the definition of ionicity regain given above, and alsofor 1 and 5 minutes shearing, and the values are shown in Table 1b.

                  TABLE 1b                                                        ______________________________________                                                Shearing Time                                                         Polymer   1 min        5 min   10 min                                         ______________________________________                                        AC         5%           9%      9%                                            BC        21%          42%     42%                                            ______________________________________                                    

It is apparent that the optimum dose for linear polymer AC, having aregain of 9%, is at 50 g/m³ whilst the optimum dose for non-linearpolymer BC, having IR 42%, is at 60 g/m³ but that polymer BC givesbetter results and its optimum results are obtained over a much widerrange, 30 to about 120 g/m³, than is permissible with polymer AC, i.e.,the non-linear polymer is less dose sensitive.

EXAMPLE 2

Five polymers were prepared by the same general process as in Example 1using the same monomers, but with differing amounts of MBA. Thepolymers, the amounts of MBA in ppm based on monomer, the intrinsicviscosity and the IR values are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Product    MBA           IV     IR                                            ______________________________________                                        GC         0             14.1   6.7                                           HC         2.5           11.1   17                                            IC         5.0           10.2   23                                            JC         10.0          6.7    42                                            KC         25.0          3.4    59                                            ______________________________________                                    

Solutions of the above products together with that of product AC wereused to treat 100 ml samples of activated sludge over a dosage range. Avisual assessment of floc size was carried out after the shearing in theTriton stirrer for 25 seconds. Using a scale of 1-8 where 1 representsthe largest and 8 the smallest floc, the results of the assessment wereplotted as shown in the attached graph. In this way, the optimum dosefor each product was obtained.

Activated sludge samples were then treated at the optimum dose with eachproduct. 200 ml portions of the treated sludge were then fed through thecentrifuge as in Example 1, when the following results were obtained.

                  TABLE 4                                                         ______________________________________                                                   Optimum Dose                                                                              Centrate Suspended                                     Product    (mg/l)      Solids (mg/l)                                          ______________________________________                                        GC         35          824                                                    HC         45          660                                                    IC         50          612                                                    JC         60          190                                                    KC         90           60                                                    ______________________________________                                    

This clearly demonstrates the improved results in high shear dewateringwhen using an increased (double) dosage of a polymer that is cros-linkedto such an extent that IR is above about 30%.

EXAMPLE 3

A rane of copolymers of 60 wt% DMAEA.MeCl and 40 wt% acrylamide wereprepared as 50% dispersions in oil as in Example 1 using differentamounts of MBA and commercially pure monomers.

All products were evaluated on a raw mixed primary activated sewagesludge as aids to gravity and compression filtration. This involvedfirst stirring the sludge with 0.1% w/v solutions of the copolymers atvarious doses, in order to optimise the dose by observing the influenceof cross linking on floc size. Further samples of sludge were thentreated at the optimum dose using periods of stirring, which representeddifferent levels of shear and allowed for the optimum development of thefloc. The stirring was by a Bosch electric drill unit fitted with amarine bladed stirrer. Dewatering was then carried out by allowing 180seconds of free drainage, on a filter wire retained in a Buchner funnel,followed by 180 seconds of drainage under compression. The filter cakeswere weighed, dried and reweighed, in order to provide a measure of drysolids content.

The results obtained were as in Table 3.

                  TABLE 3                                                         ______________________________________                                                                  Stirring                                                                              Cake                                                MBA      Dose     Time    Solids IR                                   Product ppm      (mg/l)   (seconds)                                                                             (%)    %                                    ______________________________________                                        LC      0        140      15      16.0   14.2                                 MC      2        200      45      17.1   27.6                                 NC      4        220      45      17.4   38.0                                 OC      8        300      75      20.0   50.8                                 ______________________________________                                    

EXAMPLE 4

Products PC and QC were made in the same way, and from the monomerproportions, as products GC and IC in Example 2. Additionally, a productRC was prepared at the same cationic monomer content but with 63 ppmMBA. These three additional copolymers were collected together withcopolymers JC and KC of Example 2 to form a range which increased inionicity regain and decreased in intrinsic viscosities as shown in Table4a.

                  TABLE 4a                                                        ______________________________________                                        Product  MBA (ppm)     I.V. (dl/g)                                                                             I.R. (%)                                     ______________________________________                                        PC       0             14.3       0.5                                         QC       5.0           8.4       28.0                                         JC       10.0          6.7       42.0                                         KC       25.0          3.8       59.0                                         RC       63.0          --        71.0                                         ______________________________________                                    

The above products were evaluated in the laboratory on sewage sludges inorder to determine the dose giving optimum technical performance, asdescribed in Example 2. The chosen optimum amount was then mixed into200 ml sewage sludge in a 400 ml beaker using a Heidolph Type 741.00unit fitted with a turbine stirrer for 3 minutes on a number 2 setting.The flocculated sludge was then dewatered, in simulation of beltpressing, using a piston press. This involved increasing the pressurethrough the cycle as shown in Table 4b.

                  TABLE 4b                                                        ______________________________________                                        Period of Pressing (minutes)                                                                    Pressure (bar)                                              ______________________________________                                        0-1               0.7                                                         1-2               1.4                                                         2-3               2.1                                                         3-6               2.8                                                         ______________________________________                                    

The process was conducted with two types of sewage.

On completion of the pressing cycle, the cakes were removed for drysolids determination. The results are shown in Table 4c.

                  TABLE 4c                                                        ______________________________________                                        Sludge Type                                                                             Product   Dose (mg/l) Cake Solid (%)                                ______________________________________                                        Digested  PC        250         24.8                                          primary/  QC        325         27.6                                          activated JC        375         27.1                                                    KC        650         28.1                                                    RC        1000        27.7                                          As above  PC         40         19.9                                          but       QC         60         22.5                                          containing                                                                              JC         80         23.8                                          1.0 M of  KC        150         26.3                                          added NaCl                                                                              RC        275         29.6                                          ______________________________________                                    

As can be seen from the above results, the trend is one of increasingoptimum dose and cake solids as the MBA content is increased.

EXAMPLE 5

Tests were carried out in simulation of belt pressing as described inExample 4 using products PC and JC of Table 4a at doses equal to theoptimum and also doses above and below it. Shear was applied by pouringthe flocculated sludge 10 times from one beaker to another. The digestedprimary/activated sludge used as test substrate was from an alternativesource to that of Example 4.

                  TABLE 5                                                         ______________________________________                                        Product     Dose (mg/l)                                                                              Cake Solids (%)                                        ______________________________________                                        PC          125        10.1                                                   PC          150        11.0                                                   PC          175        10.8                                                   JC          300        13.4                                                   JC          325        14.2                                                   JC          350        13.4                                                   ______________________________________                                    

The results demonstrate how the order of cake solids varies about theoptimum dose.

EXAMPLE 6

Tests were carried out in simulation of high pressure filtration (filterpressing). This involved dewatering a raw primary/activated sludge, onthe laboratory piston press at pressures of up to 7 bar. As in theprevious example, products PC and JC were evaluated at doses equal toand above and below the previously determined optimum, after shearingthe flocculated suspension by pouring from one beaker to another 15times.

Results were as shown in Table 6.

                  TABLE 6                                                         ______________________________________                                        Product     Dose (mg/l)                                                                              Cake Solids (%)                                        ______________________________________                                        PC          100        14.7                                                   PC          125        15.1                                                   PC          150        13.8                                                   JC          225        15.9                                                   JC          250        16.4                                                   JC          275        16.1                                                   ______________________________________                                    

Once again, the results demonstrate how the order of cake solids variesabout the optimum dose.

EXAMPLE 7

Three copolymers having ratios of 80 wt% DMAEA MeCl to 20 wt% acrylamidewere prepared as reverse phase suspension polymerisation dispersions.The three products contained 0, 4 and 8 ppm of MBA on weight of monomerand were identified as SC, TC and UC. Portions of products SC and UCwere mixed together to provide 50:50 and 75:25 blends respectively(SC:UC).

The two blends and the original samples were each added to a digestedPrimary/Activated sewage sludge over a dosage range and the flocculatedproduct stirred for 25 seconds on the Triton mixer of Example 1. Avisual assessment of floc size served to indicate the optimum dose foreach treatment.

Further sludge samples, treated at the optimum dose of each product andblend were evaluated on the laboratory centrifuge as described inExample 1 after vigorous stirring of the flocculated aqueous medium withthe Triton mixer. Details of the products and results obtained were asin Table 7.

                  TABLE 7                                                         ______________________________________                                                                     Optimum Centrate                                           M.B.A.   Ionicity  Dose    Solids                                   Product   (ppm)    Regain (%)                                                                              (g/m.sup.3)                                                                           (mg/l)                                   ______________________________________                                        SC        0        10.0      100     2272                                     TC        4        26.5      150     1080                                     UC        8        52.2      225      340                                     SC:UC-50:50                                                                             (4)      34.4      125     1236                                     SC:UC-25:75                                                                             (2)      22.5      125     1846                                     ______________________________________                                    

It will be observed that the enhanced performance provided by crosslinked flocculants can be obtained by blending linear and cross linkingflocculants to intermediate levels of cross linking.

EXAMPLE 8

A similar exercise to that described in the previous example was carriedout on copolymers having 60:40 wt% DMAEA MeCL:Acrylamide composition.Details of the products and results obtained are given in Table 8.

                  TABLE 8                                                         ______________________________________                                                                     Optimum Centrate                                           M.B.A.   Ionicity  Dose    Solids                                   Product   (ppm)    Regain (%)                                                                              (g/m.sup.3)                                                                           (mg/l)                                   ______________________________________                                        VC        0        14.2      100     1856                                     WC        2        27.6      150     1378                                     XC        4        38.0      175      398                                     YC        8        50.9      225      274                                     50:50: VC:YC                                                                            (4)      33.2      150      534                                     75:25: VC:YC                                                                            (2)      25.3      150     1416                                     ______________________________________                                    

EXAMPLE 9

A solid grade commercially available cationic copolymer havingcomposition 42 wt% DMAEA MeCl and 38 wt% acrylamide and two commerciallyavailable anionic copolymers having composition 10 wt% sodium acrylate:80 wt% acrylamide and 20 wt% sodium acrylate: 80 wt% acrylamide,identified as products, ZC, JA and KA respectively, were made up as 0.2%solutions. The solution of the cationic product ZC was rapidly mixed(using a Heidolph stirrer) in turn with the solutions of products JA andKA.

When mixing was insufficiently rapid, a precipitate settled.

The three compositions were evaluated on an activated sewage sludgeusing the laboratory centrifuge as described in Example 1, aftervigorous stirring using the Triton mixer for 25 seconds. The results arein Table 9.

                  TABLE 9                                                         ______________________________________                                        Centrate Suspended Solids (mg/l)                                              Dose    Solution    Solution   Solution                                       (g/m.sup.3)                                                                           ZC          80 ZC:20 KA                                                                              80 ZC:20 JA                                    ______________________________________                                        30      1000        1114       1260                                           40       632        888        1116                                           50       668        808        1048                                           60       684        408         580                                           80      1248        476         460                                           100     1820        510         320                                           125     --          668         834                                           150     1872        896        1348                                           ______________________________________                                    

It can be seen that blends of anionic and cationic solutions made fromsolid grade products are capable of producing a similar effect to thatobtained using performed cross linked polymers.

EXAMPLE 10

Using a similar procedure to that described in the previous example,solutions of product ZC were rapidly mixed with varying volumes ofsolution KA to provide blends containing 5, 10, 20 and 30% of KA.

Each solution was used to treat an activated sludge over a dosage rangeand visual assessment of floc size used to indicate the optimum dose.

Further activated sludge samples were then treated with the optimum doseof each solution before being dewatered on the laboratory centrifuge.

The following results are in Table 10.

                  TABLE 10                                                        ______________________________________                                                    Optimum Dose                                                                              Average Suspended                                     Treatment   range (mg/l)                                                                              Solids of Centrate                                    ______________________________________                                        ZC          40-60       1456                                                   5:95 KA:ZC 50-80       1400                                                  10:90 KA:ZC 50-80       1176                                                  20:80 KA:ZC  60-100     1077                                                  30:70 KA:ZC 100-140      835                                                  ______________________________________                                    

EXAMPLE 11

Settlement tests were carried out to compare product AC of Example 1 andproduct KC of Example 2 in regard to their ability to flocculate Raneycopper catalyst.

In carrying out these tests, 0.05% wt/v solutions of flocculant wereadded to 500 ml portions of 5% w/v Raney copper slurry in deionised,de-oxygenated water at room temperature. These were placed in a cylinderinverter and subjected to inversion to promote mixing after addition ofthe flocculant solution. Subsequent inversions could be carried out,following those required for mixing, in order to test the floc strength.The quality of the floc formed was, at all stages, measured in terms ofthe settlement rate of the flocculated slurry, since large flocsinvariably produce faster rates of settlement. Settlement rate wasmeasured as the time required to produce a visible mud-line in theflocculated slurry.

Tests were first carried out to determine the optimum dose of flocculantwith the results shown in Table 11a.

                  TABLE 11a                                                       ______________________________________                                        Flocculant Dose                                                                              Settlement Time (seconds)                                      (mg/l)         Product AC Product KC                                          ______________________________________                                         0             54.8       54.8                                                 1             25.1       41.8                                                10             10.4       7.8                                                 20             6.5        3.0                                                 30             4.7        1.6                                                 40             5.4        1.0                                                 50             4.5        1.7                                                 60             4.6        1.5                                                 70             4.6        1.5                                                 80             10.6       1.4                                                 90             15.4       1.9                                                 100            20.7       1.2                                                 140                       1.7                                                 180                       1.8                                                 220                       1.4                                                 260                       2.1                                                 300                       1.6                                                 340                       1.7                                                 ______________________________________                                    

From the above it can be seen that the optimum dose for each product is30 mg/l. It is, however, apparent that the overdosing effect observedfor product AC is not apparent with product KC.

Further samples were treated at the optimum dose level and subjected toinversions with settlement time being measured for each of theflocculated suspensions after equal numbers of inversions.

Results were as shown in Table 11b.

                  TABLE 11b                                                       ______________________________________                                                      Settlement Time (seconds)                                       Number of Inversions                                                                          Product AC Product KC                                         ______________________________________                                         3              6.3        1.7                                                 6              6.5        1.3                                                 9              6.0        0.9                                                12              6.7        1.2                                                15              7.8        1.0                                                18              8.1        1.1                                                21              8.8        1.1                                                24              10.5       0.9                                                27              11.3       0.9                                                30              12.6       1.4                                                33              14.4       1.3                                                36              15.9       1.3                                                39              17.3       1.7                                                42              18.9       1.3                                                45              20.8       1.2                                                60                         1.5                                                90                         1.7                                                111                        1.6                                                141                        2.0                                                171                        2.2                                                186                        2.4                                                ______________________________________                                    

From the results, it can be seen that Raney copper catalyst treated withproduct KC manifests an significantly more stable floc than that treatedwith product AC. The flocculated catalyst gave substantially the sameyield of acrylamide, when used in a conventional process for thehydrolysis of acrylonitrile, as the corresponding unflocculated catalystbut gave much easier separation of the reaction liquor from thecatalyst.

EXAMPLE 12

A range of anionic copolymers, having composition 40 wt% sodiumacrylate, 60 wt% acrylamide, were prepared from monomer mixes containingdifferent amounts of methylene bis acrylamide, by reverse phasesuspension polymerisation. The degree of structure incorporated intoeach copolymer increased in proportion to the amount of MBA in themonomer as indicated by depression of the intrinsic viscosity.

The above products were evaluated on coal fines in simulation ofdewatering by belt filtration. This involved treating 400 cm³ portionsof the coal fines with a solution of the flocculant followed by stirringfor 120 seconds to apply shear and induce flocculation. The stirring wasby a Heidolph stirrer on setting 2 using a gate stirrer in a 600 cm³beaker. The flocculated fines were then transferred to the belt presssimulator and dewatered under the influence of pressure which wasgradually increased to 1.6 bar. On completion of the dewatering cycle,the cake was removed for dry solids determination and calculation of theyield.

The MBA content, IV, results for cake solids and yield at the optimumdose established for each product are shown in Table 12.

                  TABLE 12                                                        ______________________________________                                                                           Cake                                       Pro-  MBA content            Dose  Solids                                                                              Yield                                duct  (ppm of polymer)                                                                           I.V. (dl/g)                                                                             (mg/l)                                                                              (%)   (%)                                  ______________________________________                                        AA    0            18.5      100   61.5  85.2                                 BA    2.71         14.3      150   60.0  85.8                                 CA    6.76         11.3      400   60.8  90.0                                 DA    13.53        6.6       500   57.8  86.6                                 EA    20.29        5.5       600   59.8  92.6                                 FA    27.06        2.7       800   59.8  92.4                                 GA    40.53        3.1       1200  58.6  93.7                                 HA    67.60        --        1600  59.1  89.4                                 IA    135.30       --        1600  59.7  84.9                                 ______________________________________                                    

It can be seen that as the degree of cross linking increases the generaltrend is for improvement in yield. Products HA and IA demonstratedecreasing yield either because they are too cross linked to beeffective or the optimum dose has not been attained.

EXAMPLE 13

Products AA, CA, EA and GA of Example 12 were used to treat coaltailings over a range of doses and the flocculated suspension was testedon the centrifuge as described in Example 1. The dose, in mg/l, at whichoptimum centrate quality was obtained and the suspended solids in thecentrate (%) are shown below; each result being the average of twotests, one employing 30 seconds of mixing, the other 120 seconds mixingto induce flocculation. The mixing was as in Example 12.

    ______________________________________                                                   Optimum Dose                                                                              Suspended Solids in                                    Polymer    (mg/l)      Centrate (%)                                           ______________________________________                                        A.A.       113         1.38                                                   A.C.       163         0.78                                                   A.E.       250         0.55                                                   A.G.       550         0.42                                                   ______________________________________                                    

It can be seen that as the degree of structure in the polymer isincreased (as indicated by the depressed I.V.), the optimum dose andeffectiveness increase.

EXAMPLE 14

An emulsion in oil of polymeric particles below 2 μm in size is made byreverse phase polymerisation of a blend of 40% acrylamide and 60% MeCldiethylaminoethyl acrylate and methylene bis acrylamide in an amountsufficient to raise IR from near zero to between 35 and 40.

The emulsion is added to water with stirring and allowed to age toprovide a stable composition.

The composition is then added at a polymer dose of about 6 kg/t totalsolids to an aqueous suspension that is flowing towards a commercialsewage dewatering centrifuge, the treated suspension is sheared in aInline Mixer to reduce floc size without redispersing discrete solids,and the sheared product is then dewatered in the centrifuge. The solidscontent of the centrate is typically below 0.2% (0% is ideal) and thedegree of separation is above 98% (100% is ideal). When the process isrepeated using uncross linked polymer, the corresponding values aretypically above 1% and below 75%.

EXAMPLE 15

A 1 liter resin pot containing 250 g water, 1 g Ethylan HA (non-ionicsurfactant from Lankro Chemicals Ltd.) 0.1 g V50 (polymerisationinitiator from Wako Pure Chemical Industries Ltd.) and 0.1 g Tetralon A(sequestering agent manufactured by Allied Colloids Ltd.) and bubbledwith N₂ was placed in a constant temperature water bath at 75° C.

A monomer feed was prepared by mixing 120 g of dimethyl amino ethylmethacrylate and 80 g of methyl methacrylate which was then added to amonomer feed vessel. An aqueous feed was prepared by mixing 200 g water,9 g Ethylan HA, 0.3 g V50 and 0.1 g Tetralon A which was then added toan aqueous feed vessel.

The contents of each vessel were then pumped separately, but in constantproportion, to a premixing chamber containing a high speed stirrerbefore being added to the resin pot. The pump speed was adjusted suchthat the total volume of monomer and aqueous feed was added over aperiod of 90 minutes. After the addition was complete, the same was heldat 75° C. for a period of 1 hour before being cooled.

The product of this example, designated Polymer A, was a 30% activepolymer in water. Two further samples were prepared in the manner sodescribed but containing 500 ppm and 5000 ppm of alkyl methacylate inturn. These samples were designated Polymers B and C respectively.

The products prepared as described were then diluted and acidified asthe full HCl salt to 2% active in water. Performance tests were thencarried out using the sheared CST technique as described previously on asample of digested primary/activated sludge. The results obtained aregiven in the following Table:

    __________________________________________________________________________    Cross-                                                                        linking                                                                       amount    Polymer Dose (g · m.sup.3)/Shearing time (secs)            Product                                                                             (ppm)                                                                             250/25                                                                            400/25                                                                            500/25                                                                            600/25                                                                            700/25                                                                            800/25                                                                            900/25                                                                            1000/25                                 __________________________________________________________________________    Polymer A                                                                             0 129 32  19  17  23  28  31  47                                      Polymer B                                                                            500                                                                              249 58  25  19  15  19  24  53                                      Polymer C                                                                           5000                                                                              388 140 78  67  35  25  16  17                                      __________________________________________________________________________

The results demonstrated that for these emulsion polymerised products,Polymer A (having no cross-linking agent) is susceptible to asignificant over-dosing effect above its optimum dose but with PolymersB and C, the over-dosing effect becomes less apparent as the level ofcross-linking agent increases.

EXAMPLE 16

Four solutions were prepared from a sample of 50:50 DMAEAq MeCl:ACMcopolymer (originally prepared as a 50% w/w dispersion in oil), asfollows:

Sample 1: 1% w/w active polymer with activator

Sample 2: 0.1% w/w active polymer with activator

Sample 3: 1% w/w active polymer without activator

Sample 4: 0.1 w/w active polymer without activator

All solutions were prepared in deionised water using 10 seconds lowshear mixing followed by 2 hours tumbling. The activator, when present,was an oil-in-water emulsifier.

Each sample was then used to condition aliquots of digested sewagesludge at a range of dosages, the performance being assessed in eachcase by means of CST time.

A sample of the same material, prepared as above (with activator) 24hours earlier was included as a control.

The samples containing activator (1 and 2) and the control exhibitedoptimum performance at 80-100 g/m³ and at higher doses, an over-dosingeffect was seen.

The unactivated samples (3 and 4) had optimum performance at a muchhigher dosage level (200 g/m³) and did not exhibit any over-dosingeffect, and at their optimum dose gave a lower CST (better results) thanthe CST at the optimum dose of the control and samples 1 and 2.

EXAMPLE 17

When products prepared by reverse phase polymerisation, as dispersionsin oil, are made up directly at low solution concentrations then, byvirtue of the imposed low activator concentration in the solution,activation tends to be incomplete. This results in the polymer goingonly partially into solution. On using such partial solutions, it hasbeen demonstrated that improved technical performance can be obtained.

In accordance with the above, solutions of product CA were made up atconcentrations of 1, 0.2 and 0.1% w/v active polymer. Each of thesolutions was subjected to ionicity regain determination with those at 1and 0.2% being diluted to 0.1% immediately prior to determination. Insimilar manner, the solutions were used to treat an activated sewagesludge prior to centrifugation, as described in example 1, with the 1and 0.2% concentrations being diluted to 0.1% immediately prior totreatment.

Results were as follows.

    ______________________________________                                        Original Solution    Optimum    Suspended Solids                              Concentration (%)                                                                          IR (%)  Dose (mg/l)                                                                              in Centrate (mg/l)                            ______________________________________                                        1.0          18       70        1,600                                         0.2          30      100        850                                           0.1          60      125        270                                           ______________________________________                                    

What is claimed is:
 1. A process for flocculating an aqueous suspensionof suspended solids comprising, adding to the suspension a flocculatingamount of a synthetic polymeric flocculant material to form thereby anaqueous medium containing flocculated suspended solids and in which thesaid polymeric flocculant material at the time of addition to thesuspension has a specific viscosity (measured by a capilliary flowviscometer at 34° C. on a 0.5% solution in deionised water) above 10 andcomprises polymeric particles that have a dry size of below 10 μm, thepolymeric material is added in a floc stabilising amount, and theflocculant solids are subjected to shear in the presence of the aqueousmedium substantially without increasing the amount of suspended solidsin the aqueous medium to reduce the size and increase the shearstability of said flocculated suspended solids.
 2. A process accordingto claim 1 in which the aqueous medium is then dewatered.
 3. A processaccording to claim 1 in which the flocculated solids are separated fromthe aqueous medium by dewatering selected from centrifuge, piston pressand belt press dewatering.
 4. A process according to claim 1 in whichthe flocculated solids are continuously kept in suspension by agitationof the aqueous medium.
 5. A process according to claim 4 in which theflocculated solids are catalyst particles and the aqueous medium is achemical reaction medium.
 6. A process according to claim 1 in which thepolymeric particles are of water insoluble water swellable polymer.
 7. Aprocess according to claim 6 in which the particles are of waterinsoluble, water-swellable polymer and the polymeric material is made bymixing an aqueous solution of linear water soluble polymer havingspecific viscosity above 10 with a dissolved cross linking agent whilststirring with sufficient force to form a homogeneous aqueouscomposition.
 8. A process according to claim 7 in which the crosslinking agent is a counterionic polymer having specific viscosity above10.
 9. A process according to claim 1 in which the particles are ofwater insoluble, water-swellable polymer and are cross linked and havebeen formed by polymerisation in the presence of added cross-linkingagent of a monomer or monomer blend that is soluble in the aqueouscomposition.
 10. A process according to claim 9 in which the crosslinking agent is a diethylenically unsaturated monomer and the amount ofcross linking agent is from 1 to 100 ppm based on the polymerisablemonomers.
 11. A process according to claim 9 in which the polymericmaterial has ionic regain greater than 15% and is cationic wherein saidionic regain is calculated as (x-y)/x×100 where x is the ionicitymeasured after applying standard shear and y is the ionicity of thepolymer before applying standard shear.
 12. A process according to claim9 in which the polymeric material has ionic regain of 25 to 70% and iscationic.
 13. A process according to claim 9 in which the polymericmaterial has ionic regain of 25 to 70% and is a cationic copolymer ofacrylamide with at least 5 mole percent dialkylaminoalkyl acrylate(including acid addition and quaternary ammonium salts thereof).
 14. Aprocess according to claim 9 in which the polymer has intrinsicviscosity of (100-ionic regain)/a dl/g where a is from 6 to
 14. 15. Aprocess according to claim 1 in which the polymeric particles are ofwater soluble polymer and are added to the suspension under conditionssuch that they have not fully dissolved.
 16. A process according toclaim 15 in which the particles are added to water while in the form ofa polymer-in-oil dispersion in the absence of an oil-in-wateremulsifier.
 17. A process according to claim 1 in which the particleshave been formed by emulsion polymerisation or reverse phasepolymerisation.
 18. A process according to claim 1 in which thepolymeric material is selected from materials that have intrinsicviscosity above 4 and materials that can have intrinsic viscosity above4 after shearing.
 19. A process according to claim 1 in which a 1%aqueous composition of the polymeric material that is added to thesuspension gives, if cast as a film on a glass plate and dried, adiscontinuous film of discrete swellable particles having a size ofbelow 10 μm.
 20. A process according to claim 1 in which the amount ofpolymeric material is from 50 to 150% of the amount that gives maximumfloc size after application of shear to the aqueous medium.
 21. Aprocess according to claim 1 and which comprises providing a homogeneousdilute aqueous composition of a reverse phase polymerised, nonfilm-forming, acrylamide copolymer with dialkylaminoalkyl (meth)acrylate acid salt or quaternary salt having a dry particle size below 2μm, ionic regain (IR) above 15 wherein said ionic regain is calculatedas (x-y)/x×100 where x is the ionicity measured after applying standardshear and y is the ionicity of the polymer before applying standardshear and intrinsic viscosity=(100-IR)/a where a is from 6 to 14, addingthis composition to sewage sludge in an amount of 50 to 150% of theamount required for maximum floc size after shearing, subjecting theblended mixture to the shearing to reduce floc size substantiallywithout increasing the amount of discrete suspended solids, anddewatering the resultant aqueous medium on a centrifuge, piston press orbelt press.
 22. A process according to claim 1 in which the polymericflocculant is a polymer formed from one or more ethylenicallyunsaturated monomers selected from the group consisting of acrylamide,methacrylamide, N-vinyl methyl acetamide, N-vinyl methyl formamide,vinyl acetate, vinyl pyrollidone, (meth)acrylic esters, styrene,acrylonitrile, water soluble forms of carboxylic or sulphonic acidsselected from (meth) acrylic acid, itaconic acid and 2-acrylamido methylpropane sulphonic acid, sulpho methylated acrylamide, allylsulphonate,sodium vinyl sulphonate, dialkylaminoalkyl(meth)acrylates and theirquaternary or acid salts, and dialkylaminoalkyl(meth)acrylamides andtheir quaternary or acid salts.
 23. A process according to claim 1 inwhich the polymeric material is a polymer formed by polymerisation of amonomer selected from the group consisting of water soluble acrylic acidsalt, water soluble 2-acrylamido methyl propane sulphonic acid salt,dialkylaminoalkyl(meth)acrylates and their quaternary or acid salts, anddialkylaminoalkyl(meth)acrylamides and their quaternary or acid salts,and blends of any of said monomers with acrylamide.
 24. A process inwhich an aqueous suspension of suspended solids is flocculated by addingto the suspension a flocculating amount of a synthetic polymericflocculant material to form thereby an aqueous medium containingflocculated suspended solids and in which the said polymeric flocculantmaterial at the time of addition to said suspension has a specificviscosity (measured by a capilliary flow viscometer at 34° C. on a 0.5%soluton in deionized water) above 10, and is formed by polymerization ofa monomer selected from the group consisting of water soluble acrylicacid salt, water soluble 2-acrylamido methyl propane sulphonic acidsalt, dialkylaminoalkyl (meth) acrylates and their quaternary or acidaddition salts, and dialkylaminoalkyl (meth) acrylamides and theirquaternary or acid addition salts, and blends of any such monomer withacrylamide and has been made by polymerization in the presence of addeddiethylenically unsaturated cross linking agent in an amount of from 1to 100 ppm and is in the form of polymeric particles that are waterinsoluble but water swellable and that have a dry size of below 10 μm,the aqueous medium containing the flocculated solids is subjected toshear substantially without increasing the amount of suspended solids inthe aqueous medium to reduce the size and increase the shear stabilityof said flocculated suspended solids, the amount of the polymericmaterial that is added to the suspension is from 50 to 150% of theamount that gives maximum floc size after the application of shear tothe aqueous medium, and the aqueous medium is then dewatered on acentrifuge, piston press or belt press.